To illustrate the risks, consider the following hypothetical scenario:
Jane Smith and Janet Doe have been living together for years in Texas. They have friends in Massachusetts and when they heard about the ballot initiative in Maine to legalize same-sex marriage they decided to move to New England and be a part of that political movement. They lived in Massachusetts for a year and got married in Massachusetts while traveling to Maine often to be involved in protests. After the Maine initiative passed, Jane wanted to continue protesting in other states, but Janet wanted to settled down somewhere and start a family. Realizing they had different goals Jane and Janet separated. Jane moved back to Texas. After a particularly bad winter storm, Janet decided Massachusetts wasn't for her and she moved to Florida.
While living in Texas Jane fell in love with John Lee, and they are now engaged. Jane has been told that Texas doesn't recognize her same-sex Massachusetts marriage and she is free to marry John in Texas. Jane and John get married without Jane dissolving her first marriage.
What happens if DOMA is repealed?
If DOMA is repealed then states like Texas are going to be required to recognize out-of-state marriages even if they are same-sex marriages. At that point Jane will have two marriages recognized by Texas law. This could possibly make her second marriage void. It also might create a violation of the bigamy laws in Texas, because Jane will be married to two people and still living with her second spouse.
What happens if Jane and John move to a state that recognizes same-sex marriages, like Maine?
A state like Maine which allows same-sex marriage would recognize the original marriage. At that point Jane will have two marriages recognized by her state of residence. This could possibly make her second marriage void. It also might create a violation of the bigamy laws in Maine, because Jane will be married to two people and still living with her second spouse.
How can Jane avoid these problems?
Since Jane and Janet last lived together in Massachusetts, they can file for divorce in Massachusetts and dissolve the first marriage. Once complete this will allow Jane to remarry without having to worry about the consequences.
However, if we change one fact in this hypothetical another problem arises. If Jane and Janet didn't move to Massachusetts but just traveled there to get married while remaining residents of Texas, then they are are married in any state that recognizes same-sex marriage. However, they cannot get divorced in Massachusetts because neither is a resident and they never lived together in Massachusetts. This means that Jane can't give divorced without moving to a state that recognizes same-sex marriages long enough to meet the residency requirements.
In this scenario, Jane can't get divorced and she shouldn't get re-married without getting divorced first. Was it the intention of DOMA's drafters to prevent marriages between opposite-sex couples in their states? Probably not, however, often poorly written legislation has unintended consequences.